DIP 1005 proposes a solution to prevent loading in modules that
don't need to be loaded, thereby decreasing the overall compile
time. Here's an example taken from the DIP:
with (import std.stdio) void process(File input) ;
with (import std.range) struct Buffered(Range) if
(isInputRange!Range)
{
...
}
I've come up with another way to solve this problem by using a
modified version of import, call it a "lazy import". A lazy
import works like a "static import" in that all references to it
must be fully qualified, but it also delays loading the module
until absolutely necessary. Since all the symbols will be fully
qualified the semantic analyzer will know when it's time to load
the module. The previous example could be done like this:
lazy import std.stdio;
lazy import std.range;
void process(std.stdio.File input) ;
struct Buffered(Range) if (std.range.isInputRange!Range)
{
...
}
Note that instead of introducing the "lazy" modifier here, we
could just modify static imports to be lazy which would mean no
new syntax and every benefits from the feature without changing
their code, that is, if they are using static imports :)
aliasing can also be helpful in this instance
alias stdio = std.stdio;
alias range = std.range;
void process(stdio.File input) ;
struct Buffered(Range) if (range.isInputRange!Range)
{
...
}
Note that this usage is only meant for large library code like
phobos, where this type of optimization can have a big
performance impact when importing modules from the library. You
wouldn't use the verbose syntax from either of the examples in a
normal application.
What do you think, better or worse than DIP 1005? I think it's
simpler, but not sure if it's better overall.

Wanted to add that I believe we could also make "selective
imports" lazy, either be default or possibly by adding a modifier
like "lazy" if non-lazy imports are still useful.
lazy import std.stdio : File;
lazy import std.range : isInputRange;
void process(File input) ;
struct Buffered(Range) if (isInputRange!Range)
{
...
}
It works because the semantic analyzer knows that the "File"
symbol is referring to something in module std.stdio, so it can
delay importing std.stdio until it needs to resolve the File
symbol.
If this was implemented, not sure if there is a benefit to
turning lazy imports on/off, if not, then we could just forgo the
new "lazy" modifier and just make all "qualified imports" lazy.
Note this can't work with normal imports because in that case you
need to import the module just to know what symbols it contains.
This idea only works with "static imports" and "selective
imports".

Note that instead of introducing the "lazy" modifier here, we
could just modify static imports to be lazy which would mean no
new syntax and every benefits from the feature without changing
their code, that is, if they are using static imports :)

That DIP already has a section titled "Alternative: Lazy Imports"
that discusses this idea, also in context of static imports.

Note that instead of introducing the "lazy" modifier here, we
could just modify static imports to be lazy which would mean
no new syntax and every benefits from the feature without
changing their code, that is, if they are using static imports
:)

That DIP already has a section titled "Alternative: Lazy
Imports" that discusses this idea, also in context of static
imports.

Lol, that's exactly what I was suggesting :) Thanks for pointing
that out.

What do you think, better or worse than DIP 1005? I think it's
simpler, but not sure if it's better overall.

Walter has talked before about wanting to make _all_ imports lazy, which
should obviate the need for DIP 1005 entirely - at least as far as
compilation speed goes. The benefit that you do not get is the documenting
of dependencies that DIP 1005 gives you. But personally, I don't think that
that's even vaguely worth complicating function signatures even further -
especially if we have lazy imports. I would _much_ rather have lazy imports
implemented rather than DIP 1005. Function signatures arguably have too much
on them in D as it is.
- Jonathan M Davis